With the development of motor drive systems in recent years, frequency control of the driving power source has become possible, and motors increasingly operate at variable speed or rotate at high speed at or above commercial frequency. In such motors that rotate at high speed, the centrifugal force acting on the rotating body, such as a rotor, grows large proportional to the radius of rotation, in proportion to the square of the rotational speed. Hence, high-strength material needs to be used as the rotor material, in particular for a medium or large-sized high-speed motor.
Furthermore, in recent years, slits in which magnets are embedded are provided along the outer periphery of the rotor in an IPM (Interior Permanent Magnet)-type DC inverter control motor, which is increasingly being used as, for example, a drive motor or a compressor motor in a hybrid vehicle. Therefore, due to the centrifugal force during high-speed rotation of the motor, stress concentrates on the narrow bridge (such as the portion between the rotor periphery and the slits). Moreover, since the stress state varies due to acceleration and deceleration of the motor and due to vibration, the core material used in the rotor not only needs to be high strength but also needs to have a high fatigue strength.
Additionally, in a high speed motor, eddy current occurs due to high-frequency magnetic flux, both reducing the motor efficiency and generating heat. When the amount of generated heat grows large, the magnets embedded in the rotor are demagnetized. A low iron loss in the high-frequency range is thus also desired.
Accordingly, as the material for a rotor, there is a demand for an electrical steel sheet that has excellent magnetic properties and is high strength.
Solid solution strengthening, strengthening by precipitation, strengthening by crystal grain refinement, and multi-phase strengthening are known as methods for strengthening steel sheets, yet many of these strengthening methods cause magnetic properties to degrade. Therefore, typically it is extremely difficult to make strengthening compatible with magnetic properties.
In such circumstances, several proposals have been made regarding high-tension electrical steel sheets.
For example, JP S60-238421 A (PTL 1) proposes a method for strengthening by increasing the Si content to 3.5% to 7.0% and adding elements such as Ti, W, Mo, Mn, Ni, Co, and Al for solid solution strengthening.
In addition to the above strengthening method, JP S62-112723 A (PTL 2) proposes a method for improving magnetic properties by setting the crystal grain diameter to 0.01 mm to 5.0 mm by adjusting the final annealing conditions.
When these methods are applied to factory production, however, problems such as sheet breakage occur more easily in the continuous annealing step after hot rolling, the subsequent rolling step, and the like, leading to problems such as reduced yield or the need to shut down the production line.
To address this issue, performing warm rolling with a sheet temperature of several hundred ° C. instead of cold rolling reduces sheet breakage. Equipment for warm rolling becomes necessary, however, and serious process control problems also occur, such as severe restrictions on production.
JP H02-022442 A (PTL 3) proposes a method for solid solution strengthening that adds Mn or Ni to steel with a Si content of 2.0% to 3.5%. JP H02-008346 A (PTL 4) proposes a technique for making high strength compatible with magnetic properties by solid solution strengthening that adds Mn or Ni to steel with a Si content of 2.0% to 4.0% and furthermore by using carbonitrides of Nb, Zr, Ti. V, and the like. JP H06-330255 A (PTL 5) proposes a technique for making high strength compatible with magnetic properties by using a precipitation effect and a grain refinement effect due to carbonitrides of Nb, Zr, Ti, V and the like in steel with a Si content of 2.0% or more and less than 4.0%.
These methods, however, have the problem of a higher cost because of the addition of a large amount of expensive elements, such as Ni, and because of reduced yield due to an increase in defects such as scab. Furthermore, these disclosed techniques use a precipitation effect due to carbonitrides and therefore have the problem of a large degradation in magnetic properties.
JP H04-337050 A (PTL 6) discloses a technique for increasing steel sheet strength by setting the recrystallization rate of the crystallized microstructure to be 95% or less and the balance to be effectively a rolled microstructure by heat treatment, at a specific temperature prescribed by the relationship with the Si content, of a cold-rolled steel sheet having a chemical composition of Si: 4.0% to 7.0%.
With the above technique, for example when performing heat treatment at 700° C., it becomes necessary to add at least approximately 5.9% of Si, yet the result is a practical soft magnetic material that has tensile strength as high as 80 kgf/mm2 or more and a desired elongation and that is also provided with excellent magnetic properties.
For an electrical steel sheet that includes Si: 0.2% to 4.0% and has a ferrite phase as the main phase, JP 2005-264315 A (PTL 7) discloses a technique for increasing steel sheet strength by adding Ti, Nb, Ni, and the like and by generating intermetallic compounds having a diameter of 0.050 μm or less inside the steel material. With this method, a non-oriented electrical steel sheet that has tensile strength of 60 kgf/mm2 or more, abrasion resistance, and excellent magnetic flux density and iron loss properties can be manufactured without detriment to cold rolling manufacturability or the like.
Furthermore, JP 2005-113185 A (PTL 8), JP 2006-169611 A (PTL 9), and JP 2007-186790 A (PTL 10) propose a high-strength electrical steel sheet in which a non-recrystallized microstructure is made to remain in the steel sheet. With these methods, a high strength can be obtained relatively easily while maintaining manufacturability after hot rolling.
Each of these materials, however, has the problem of variance in the steel sheet strength tending to increase in the direction orthogonal to the rolling direction.
Therefore, JP 2010-090474 A (PTL 11) proposes a method for producing a high-strength non-oriented electrical steel sheet using a slab for which the chemical composition is adjusted to include Si: over 3.5% and 5.0% or less, Al: 0.5% or less, P: 0.20% or less, S: 0.002% or more and 0.005% or less, and N: 0.010% or less, and adjusted so that Mn is in a range satisfying the following relationship with respect to the S amount (mass %):(5.94×10−5)/(S %)≤Mn %≤(4.47×10−4)/(S %).
With this technique as well, however, the variation in steel sheet strength cannot be considered to be at a desired value for actual use. As before, demand exists for an electrical steel sheet that has low iron loss and that exhibits little variation in strength while being high strength.